We visited Basílica de San Isidoro, León and spent an hour learning some amazing things, seeing one of the Christian holy grails (pretty special), visiting the crypts and seeing the most amazing 700 year old fresco's, but the thing that stuck in my mind was the library. The books they have are hundreds and hundreds of years old but not only that, they are HUGE. Some weigh as much as 30 kilo and the pages are actually calf skin. The ilustrations are incredible and the penmanship of the monks is indescribable. It remains on of the best religious sites I've ever done. Holy Grail - tick!